Regime transitions in drag reduction by air lubrication*

Friction drag accounts on average for approximately 70% of the overall resistance of a ship, and thus a large part of a typical ship’s propulsive power is required to overcome it. To reduce this drag air layer lubrication techniques have been proposed and investigated over the past years. These techniques could lead to fuel cost savings and a lower environmental impact. While drag reduction measurements using a variety of injectors and flow conditions have been performed by different research groups to date, no reliable scaling laws are available. Depending on the liquid freestream velocity, the air injection rate, and the injection method, different air phase regimes are commonly observed: a bubbly regime, a transitional regime and an air layer regime. The onset of the air layer regime is particularly important since this is the desired regime in terms of drag reduction. The present study aims to investigate regime transitions via comparing experimental data from various experimental facilities resulting in a wide range of Reynolds and Froude numbers. The imaging procedure to acquire and quantify these data is described and preliminary conclusions are drawn on the important parameters taking into account the real-life application of this technology.